Methods for lambda control in internal combustion engines can be used to reduce the emissions of harmful exhaust gases into the environment. For this purpose, at least one catalytic converter can be disposed in the exhaust system of the internal combustion engine. In order to keep the catalytic converter at an optimal operating point, it is necessary to control the mixture preparation of the internal combustion engine using a lambda controller such as to give a regulated lambda number that is very close to 1.0 at least on average. A lambda probe can be disposed in the exhaust system of the internal combustion engine for generating a measurement signal.
The prior art is inter alia the use of one of the two control methods described below.
A control method is illustrated in FIG. 2 as it is normally applied when using a step change lambda probe. The upper graph shows the probe signal against time and the lower graph shows the controller intervention against time. With said probes the direction of the controller is changed if the probe signal crosses a specified threshold, for example 450 mV, which in this case corresponds to the stoichiometric point (in this case at times t1, t2 and t3). The variation of the signal above or below the respective threshold is not used or exploited further during the control, but the adjustment takes place independently thereof by pre-control, generally by means of a specified P-component and an I-component, which in turn can be dependent on other variables such as for example the operating point.
The relatively slow control rate is disadvantageous with this method, because above or below the control threshold the absolute signal value is not considered further and thus even large mixture deviations are only corrected at the previously determined control rate. Furthermore, it is a disadvantage that the changeover frequency is relatively high and essentially only determined by the path transition time to the probe and the dead time of the probe. There is thus no possibility of definitely specifying the oxygen input to or output from the downstream catalytic converter, so that the conversion efficiency of the catalytic converter is limited.
FIG. 3 illustrates a control method as normally applied when using probes with accurate lambda signals, including away from the stoichiometric point, i.e. generally broadband lambda probes (actual lambda number from the probe signal: bold dark curve; target lambda number at the probe: narrow dark curve; control variable of the controller: bold light curve; target engine lambda number: narrow rectangular wave curve). The modulation is adjusted by means of varying the target lambda number. The control error is determined from the difference between the target value and the measured actual value and is fed to a suitable controller (for example a PID controller). The path characteristic is taken into account if the target engine value is not used for difference computation but the profile of the target engine value is based on the position of the probe, taking into account the path transition time, and said value is used as the target value at the probe position.
The advantage of this method is that the desired lambda number can be set accurately and the controller has a rapid control rate. It is a disadvantage that overshoots of the controller and strong fluctuations of the fuel-air mixture can occur if the stored path characteristic does not agree with the actual path dynamics. This is the case for example if the probe becomes dynamically more sluggish through ageing or contamination. This is illustrated by way of example in FIG. 4 (actual lambda number from the probe signal: bold dark curve; control variable of the controller: bold light curve; target engine lambda number: narrow rectangular wave curve). In this case the probe signal is significantly more sluggish than in FIG. 3. At point in time t1, when the probe signal reaches the target value, the control value has therefore already changed significantly and as a result there are overshoots in the controller and in the lambda number (point in time t2), and the target value can only be regulated to be stable after a delay (point in time t3). This is a disadvantage for the efficiency of the downstream catalytic converter, i.e. increased emissions occur, with greater fluctuations in the fuel-air ratio this can also cause noticeable juddering of the engine.
If the lambda signal is determined from the signal of a step change lambda probe, a controller according to FIG. 3 has yet another disadvantage. A typical characteristic of step change lambda probes is illustrated in FIG. 5. The step change region can be seen, i.e. the region of large signal change, in the region where lambda=1. Current probes respond dynamically more sluggishly in this step change region than in the pure rich or pure weak region. A lambda signal computed from a step change probe signal therefore has a time delay at a change of mixture between rich and weak exhaust gas for the lambda=1 region. This is to be seen in FIG. 4 at the point in time t4. This behavior also leads to overshoots in the control value and as a result in the lambda number for this type of controller, as illustrated at the point in time t5, with the disadvantages described above. Alternatively, the control parameters could be adapted to the reduced dynamics at the lambda=1 point, but the controller would then be significantly slower in the region outside the lambda=1 region than it could actually be.
An approach is already known from DE 10 2006 049 656 A1 as to how advantages of the method in FIG. 3 illustrated can be exploited for probes with inaccurate correlation between the signal and the actual mixture composition in the region away from the stoichiometric point (thus for example step change probes), in which according to the prior art the method illustrated in FIG. 2 is used. It is described there how a changeover of the controller direction only takes place if a probe signal not only exceeds or falls below a signal threshold value, but also a threshold value for a variable derived from the probe signal. This enables a defined oxygen input or output into or out of the catalytic converter to be provided with known accuracy and thus the conversion efficiency of the catalytic converter to be increased. However there remains the disadvantage of the slow correction of mixture deviations.